Saying act 3 nightmare implies you have played through completely once, which further implies that the game itself is not the issue.

In addition some more details on the crash may help, does your PC restart, freeze up, have you got a temperature monitoring software solution (MSI Afterburner) checking your temperatures?

Can you use any other games for longer than 5 minutes?

I know that D3 is a little unstable - i use a GTX480 and i get FPS drop but i highly doubt that the game itself is causing the crashes - the fact you have played through once at least implies that too, nothing has changed with the software from normal to nightmare just the difficulty setting, which, if this is causing your issues would not have waited till Act 3 to show itself.

I'm not in any way defending anything or anyone here, just trying to help with your PC issue (if i can)

Did you recently start nightmare mode? The reason I ask is that Nvidia did release a new driver a few days ago which has been hit and miss for a lot of people, myself included. Is it at all possible that you starting nightmare mode coincided with the new driver update from Nvidia?Have you tried rolling back to driver version 296.10 to see if the issue persists?